Web Summary Feeds with a soft, moist texture have applications in feeding shrimp and fish. Extrusion technology can be used to produce such feeds. This article explains the choice of equipment, additives and operating conditions to produce soft-moist feeds.
Soft-moist feeds can have several advantages over the dried pelleted feeds. For one, the soft product may be more easily consumed by the crustaceans. To eat, crustaceans take a pellet in their front claws and shred it into small pieces. As it shreds, the other legs make or knead the shreds into a ball that is guided to its mouth. If the pellets are too hard the shrimp will need to wait until it has expanded by the water action to consume it. In some cases the shrimp takes it but after trying to consume it without being successful it drops it to look for another one. Soft moist feeds could also have better water stability than regular pelleted feeds or other extruded feeds. In the pelleting process the binding of the particles occurs as the feed is heated and wetted in the long retention time pre-conditioners or by the action of synthetic binders. With high shear extruders the starch is degraded, or sheared to the point that it looses its binding ability and this is exacerbated by an increase in its water absorption capacity to the point that when the pellet is submerged it swells and disintegrates. In contrast, the low shear rate of the soft-moist extrusion process involves the right conditions of moisture (high), shear (low), and temperature (above gelatinization) to produce a pellet that is soft and has high water stability. The soft moist extrusion process can permit the inclusion higher volumes of liquids, such as hydrolysates, and solubles than cannot be incorporated, due to moisture limitations, in the pelleting process. In the case of liquid ingredients such as hydrolysates and solubles with low pH, their higher inclusion could contribute to prevent mold growth. Soft moist feeds can also be easily manipulated to produce rounded shapes using vibratory equipment. The latter cannot be done with the traditional pelleting press because the product is already too hard. The equipment that can impart the sphericity to the soft moist feed is known as a marumerizer or spheronizer. Soft moist feeds will also have applications in feeding fish. They can be used in weaning wild-caught broodstock of carnivorous fish that eat only live prey or a feed high in fresh fish. They can also be used to feed species that eat slow and prefer a feed with soft texture. This article has one table and one figure. Full Paper
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A key consideration in the production of soft-moist feeds is the application of low specific mechanical energy which is provided by a screw profile with mostly forward conveying elements that are long pitch, and are rotated at low screw speeds. The die should also be selected with care to avoid excessive shear. Use flex-type blades with very sharp cutting edge so that the shape of the particle is not distorted. Soft-moist products require conveying systems that can handle the product gently. Therefore, the recommended type of conveyor is the belt conveyor. During conveying and handling the product should not be piled up or keep the bed depth as low as possible to avoid damaging the soft pellets with its own weight. Since the product must remain soft, it is not dried but rather cooled to minimum of 10C above ambient temperature. The preferred unit for cooling soft moist products is a horizontal unit to prevent product packing and damage. In some cases the horizontal coolers are designed with moisture control units to prevent excessive loss of moisture from the material. In the packaging area the product must be stored in horizontal bins with a maximum bed depth of 2 meters, again to prevent product packing and damage.
Water acts as a lubricant and plasticizing agent in the feeds. As a lubricant it reduces the mechanical energy input, which results in a lower temperature process. Because the temperature and pressure are low, the drop in pressure as the dough exits the die is not drastic and this results in little expansion of the product. Therefore, the product may end up with moisture content of around 20% but in most cases as high as 28%. At this moisture levels the major concern is to prevent microbial growth and this can only be achieved by controlling the water activity (Aw). Keeping Aw as low as possible and below 0.65 is important to prevent microbial growth and extend the shelf life and freshness of the product. This is best done by the inclusion of 10-15% of humectants or ingredients that have the property to bind water. Such ingredients include sugars, salts, propylene glycol, honey, corn syrups, disaccharides, glycerin, and maltodextrins. Another way to control mold growth in the final product is by reducing the pH. Acids such as lactic, propionic or phosphoric can be used to reduce the pH. Also mold inhibitors are used in the formula. A common ingredient used as mold inhibitor is potassium sorbate (0.2-0.3% inclusion).
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